Isopoda - Woodlice, aquatic species, etc.
Crustacea (water fleas, cyclops, barnacles, crabs, shrimps, etc.)
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On the left and below are woodlice, also known as sow-bugs or slaters, Order Isopoda. In the Crustacea phylum they are the only order that contain species that are wholly terrestrial. Although there are also freshwater (see Ascellus below) and marine species. There are over 4 000 species of isopods in the world, the British and Irish woodlice are listed below, most are small, although there is a deep sea giant aptly named Bathynomus giganteus, which can reach 42 cm long and 15 cm wide! Most are grey or grey/brown in colour. In the terrestrial species the animal's whole life revolves around the avoidance of desiccation. Therefore it is active mainly during the night. It is thought that isopods colonised the land during the Carboniferous.

On the left you can see the underside of a terrestrial woodlouse, they have 7 pairs of walking legs. The first pair of antennae are usually short, and in terrestrial species they are often vestigial. The second pair of antennae are usually well developed, except in the parasitic species.

The female carries her eggs and young in a fluid-filled pouch beneath her), and it takes about two years for the young to reach maturity.

Oniscus asellus, the common shiny woodlouse (left) is often found under logs and in compost heaps. It is shiny grey with lighter grey patches, and its eyes consist of a number of black ocelli. It can reach 15 mm long and 7.5 mm wide, and is probably the most common British woodlouse, and is found anywhere there are damp conditions, it is also common in north and western Europe, is present in eastern Europe and noth America. It remains motionless when disturbed, but can run quite fast.

There are 37 species of woodlice native to the British Isles (see the list below), but there are a few introduced species which can breed indoors only. Most range in size from 5 - 15 mm. On release from the female's brood pouch the young woodlouse has 6 pairs of legs. Within 24 hours it will moult, and the 7th segment which will bear legs appears. After the next moult it will have the full compliment of 7 pairs of legs. Very few woodlice live longer than 2 - 3 years, however in California, where the common pill bug Armadillidum vulgare (see below right) was introduced just over 100 years ago, it lives for 4 years, and females can have 3 or 4 broods.

UK woodlice (and most others) feed mainly on rotting vegetation, and so help to return valuable nutrients to the soil. Woodlice rarely eat living plants, so gardeners should not consider them pests. However they do take an occasional nibble of seedlings in greenhouses and cold frames. On the whole they do much more good than harm, and are especially useful in chewing up plant fragments in compost heaps, and their faeces aids decomposition. Although UK woodlice are vegetarian, there are a few others that are not. There is one (Scyphax ornatus) in New Zealand which lives on sandy beeches, and specialises in eating drowned honey bees. And in the USA woodlice are used in museums to clean the flesh off vertebrate skeletons.

They can exude a repellent secretion from their rear end to warn off predators.

In olden days woodlice were carried around in a small bag and used to treat stomach aches. As their exoskeleton is mainly calcium carbonate they may have been able to neutralise stomach acids and so treat ulcers, heartburn, and over indulgence in general.

Woodlice are preyed upon by spiders, toads, and centipedes.

Left is Porcellio spinicornis, a particularly attractive woodlouse. Fully grown it reaches about 12 mm long. It has a black head, black eyes, a dark central stripe with a row of yellow blotches down either side. It can run quite fast, but usually stays still for a while when first disturbed. It is found in walls and buildings especially those with lime-rich mortar. It is very common in north east Scotland and in drystone walls in the Cotswolds, also northern France, northern Italy, Russia, Canada, and the USA.

On the right is Ligia oceanica, the Common sea slater. When fully grown it can reach 30 mm long making it the largest British woodlouse. Note its relatively long antennae. It is found only on sea shores, but is a fairly common sight around Britain and Ireland in crevices on rocky shores, emerging at night to graze on seaweed and algae. Sometimes it is found on the strandlines on beaches. Note that all 4 uropods are the same length. It is greenish-grey to a light grey-brown, and matches the background. It can change colour by expanding and contracting special melanin containing cells located just under its cuticle, and is used to study colour change in Crustacea. It has fairly large eyes consisting of numerous ocelli. When disturbed it can run quite quickly. It is a "popular" food on survival courses!

Left is Platyarthrus hoffmannseggi, the ant woodlouse. It is blind, white and found in ants nests in Wales and southern England, it is less common in northern England, and absent from Scotland, but it is fairly widespread throughout Europe. In Ireland it is recorded only in the south east. It is less than 4 mm long. Sometimes the contents of its gut can be seen through the cuticle.

Armadillidium vulgare, the pill bug (right), can tolerate dry places, also its ability to roll up helps it conserve moisture. It can be found in grassy places, especially chalky and limestone areas, and is often confused with the pill millipede, but can be distinguished by its numerous small rear segments. When fully

grown it can be up to 1.8 cm long. It is variable in colour, though most are light grey, but pink, black and yellow individuals have been seen. It is common in S E England and Ireland, throughout Europe, parts of Asia, North America, Australasia, South Africa and some Pacific and Atlantic islands. The pill bug was once used by doctors - that's where it gets its name- patients had to swallow one whole!

As is the case in European wiidlice, the eggs are laid and brooded in a pouch formed by the plates arising from the 2nd - 5th thoracic appendages. On hatching the young have just 6 pairs of legs, and are pale-coloured. They acquire the 7th pair of legs after their first moult, and by this time have left the brood pouch. Moulting occurs throughout their life, and usually takes place in crevices in the soil. They moult one half of the body at a time (see the Porcellio scaber photograph), with the rear half being moulted first. It can be as long as 2 weeks before the front half is moulted.